Relationship design patterns in GraphQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a One-to-Many Relationship Query

Given a GraphQL schema where Author has many Books, what is the output of this query?

query {
  authors {
    name
    books {
      title
    }
  }
}

Assume the data has one author named "Alice" with two books titled "GraphQL Basics" and "Advanced GraphQL".

GraphQL

query {
  authors {
    name
    books {
      title
    }
  }
}

A {"data":{"authors":[{"name":"Alice","books":[]}]} }

B {"data":{"authors":[{"name":"Alice"}]}}

C {"data":{"authors":[{"name":"Alice","books":[{"title":"GraphQL Basics"},{"title":"Advanced GraphQL"}]}]}}

D {"errors":[{"message":"Field 'books' not found on type 'Author'"}]}

💡 Hint

Think about how one-to-many relationships return nested lists in GraphQL.

The query requests each author's name and their list of books with titles. Since Alice has two books, both appear nested under her.

🧠 Conceptual

intermediate

Identifying Relationship Types in GraphQL Schema

Which of the following GraphQL field definitions best represents a many-to-many relationship between Student and Course?

A type Student { courses: [Course!]! } and type Course { students: [Student!]! }

B type Student { course: Course } and type Course { student: Student }

C type Student { courses: Course } and type Course { students: Student }

D type Student { courses: Course! } and type Course { students: Student! }

💡 Hint

Many-to-many means each side can have multiple related items.

Option A uses lists on both sides, indicating multiple courses per student and multiple students per course, which is many-to-many.

📝 Syntax

advanced

Correct Syntax for One-to-One Relationship Field

Which option shows the correct GraphQL syntax for a one-to-one relationship where a User has one Profile?

A type User { profile: Profile! }

B type User { profiles: [Profile] }

C type User { profile: [Profile!]! }

D type User { profile: Profile }

💡 Hint

One-to-one means exactly one related item, not a list.

Option A declares the profile field as non-nullable single Profile, matching one-to-one.

❓ optimization

advanced

Optimizing Nested Relationship Queries

You have a GraphQL query fetching Posts with their Comments and each comment's Author. Which approach reduces over-fetching and improves performance?

A Fetch all fields of Posts, Comments, and Authors without selection sets.

B Use fragments to select only needed fields for Comments and Authors.

C Fetch Posts only and query Comments and Authors in separate queries.

D Fetch Posts with Comments but omit Authors to reduce data.

💡 Hint

Fragments help reuse and limit fields fetched.

Using fragments to select only necessary fields avoids fetching unnecessary data and improves query efficiency.

🔧 Debug

expert

Debugging Circular Relationship Query Error

Given a GraphQL schema where Employee has a manager who is also an Employee, this query causes an error:

query {
  employees {
    name
    manager {
      name
      manager {
        name
      }
    }
  }
}

What is the most likely cause of the error?

A The query syntax is invalid because nested fields are not allowed.

B The 'manager' field is not defined on Employee type.

C The 'name' field is missing from the Employee type.

D The query causes infinite recursion due to circular references without depth limit.

💡 Hint

Think about how recursive relationships can cause problems in queries.

Querying nested managers repeatedly without limiting depth can cause infinite recursion or stack overflow errors.

Practice

1. In GraphQL, which relationship pattern describes one item connected to many items, like one author having many books?

easy

A. One-to-many

B. Many-to-one

C. Many-to-many

D. One-to-one

Solution

1. Step 1: Understand relationship types

   One-to-many means one record links to many records, like one author to many books.

2. Step 2: Match example to pattern

   Since one author can have many books, this fits one-to-many.

3. Final Answer:

   One-to-many -> Option A

4. Quick Check:

   One author -> many books = One-to-many [OK]

Hint: One item linked to many items means one-to-many [OK]

Common Mistakes:

• Confusing many-to-many with one-to-many
• Thinking one-to-one fits multiple items
• Mixing up direction of relationship

2. Which of the following is the correct GraphQL syntax to define a many-to-many relationship between Student and Course types?

easy

A. type Student { courses: Course }
type Course { students: Student }

B. type Student { course: Course }
type Course { student: Student }

C. type Student { courses: [Course!]! }
type Course { students: [Student!]! }

D. type Student { courses: Course! }
type Course { students: Student! }

Solution

1. Step 1: Identify many-to-many syntax

   Many-to-many uses lists on both sides, so fields must be arrays like [Course!]! and [Student!]!.

2. Step 2: Check options for correct list syntax

   type Student { courses: [Course!]! } type Course { students: [Student!]! } uses lists with non-nullable items and non-nullable lists, which is correct GraphQL syntax.

3. Final Answer:

   type Student { courses: [Course!]! } type Course { students: [Student!]! } -> Option C

4. Quick Check:

   Lists on both sides = many-to-many syntax [OK]

Hint: Many-to-many uses lists on both types in GraphQL [OK]

Common Mistakes:

• Using single objects instead of lists for many-to-many
• Missing non-nullable markers (!)
• Confusing one-to-one syntax with many-to-many

3. Given this GraphQL schema snippet:

type Author {
  id: ID!
  books: [Book!]!
}

type Book {
  id: ID!
  title: String!
  author: Author!
}

What will the query { author(id: "1") { books { title } } } return if author 1 has two books titled "A" and "B"?

medium

A. {"data":{"author":{"books":{"title":"A"}}}}

B. {"data":{"author":{"books":[{"title":"A"},{"title":"B"}]}}}

C. {"data":{"author":{"books":null}}}

D. {"errors":[{"message":"Field 'books' not found"}]}

Solution

1. Step 1: Understand schema and query

   Author has a list of books. Query asks for author with id "1" and their books' titles.

2. Step 2: Predict query result

   Since author 1 has two books "A" and "B", the books field returns a list with both titles.

3. Final Answer:

   {"data":{"author":{"books":[{"title":"A"},{"title":"B"}]}}} -> Option B

4. Quick Check:

   List of books with titles A and B returned [OK]

Hint: List fields return arrays of objects in GraphQL query results [OK]

Common Mistakes:

• Expecting a single book object instead of a list
• Assuming null if multiple books exist
• Confusing error with missing field

4. Consider this GraphQL type definition:

type User {
  id: ID!
  posts: Post
}

type Post {
  id: ID!
  content: String!
}

What is the main problem with this design if a user can have many posts?

medium

A. The 'posts' field should be a list, not a single Post object.

B. The 'id' field is missing in User type.

C. The 'content' field should be nullable.

D. The Post type should include a reference back to User.

Solution

1. Step 1: Analyze the 'posts' field type

   The 'posts' field is declared as a single Post, but users can have many posts.

2. Step 2: Identify correct type for multiple posts

   To represent many posts, 'posts' should be a list like [Post!]! instead of a single Post.

3. Final Answer:

   The 'posts' field should be a list, not a single Post object. -> Option A

4. Quick Check:

   Multiple posts need list type, not single object [OK]

Hint: Multiple related items need list type in GraphQL [OK]

Common Mistakes:

• Thinking single object can hold many items
• Ignoring list syntax for multiple relations
• Assuming missing fields cause this error

5. You want to model a social network in GraphQL where users can follow many other users and be followed by many users. Which relationship design pattern and schema snippet correctly represents this many-to-many self-relationship?

hard

A. type User { follower: User following: User } // single user on each side

B. type User { followers: [User] following: User } // mixed list and single user

C. type User { followers: User! following: User! } // non-null single user

D. type User { followers: [User!]! following: [User!]! } // both lists represent many-to-many

Solution

1. Step 1: Understand many-to-many self-relationship

   Users can follow many users and be followed by many users, so both sides need lists.

2. Step 2: Check schema for correct list usage

   type User { followers: [User!]! following: [User!]! } // both lists represent many-to-many uses lists for both followers and following, correctly modeling many-to-many self-relationship.

3. Final Answer:

   type User { followers: [User!]! following: [User!]! } -> Option D

4. Quick Check:

   Lists on both sides model many-to-many self-relationship [OK]

Hint: Self many-to-many needs lists on both fields [OK]

Common Mistakes:

• Using single User instead of list for followers/following
• Mixing list and single object types
• Forgetting non-nullable list syntax